Lithium-ion batteries are currently the most popular rechargeable batteries due to their high energy densities, relatively high cell voltages, and low weight-to-volume ratios. However, the voltage, charge capacity, battery life, and rechargeability of lithium-ion batteries have increased by relatively small increments over the past decade.
Graphite is the primary active material in commercial lithium-ion battery anodes, and the theoretical maximum gravimetric energy capacity of these materials (372 mAh/g) is approached in most lithium-ion battery anodes on the market today. Anode manufacturing methods have been designed to effectively process and incorporate graphite into batteries, particularly in lithium ion batteries. In addition, there are many valuable features of graphite, including, for example, high Coulombic efficiency and tolerance of high cycling rates that are useful to retain in an anode formulation. On the other hand, attempts to utilize higher capacity materials, often in the form of submicron particles or nanoparticles, in lithium ion battery anodes have met with numerous problems associated with severe agglomeration of the particles, unstable connectivity with electronically conducting particles, high volumetric expansion during lithiation, and poor Coulombic efficiency due to side reactions with the electrolyte and unfavorable solid electrolyte interface (SEI) formation. Various expensive processes have been designed to overcome these problems, including, chemical vapor deposition (CVD) of carbon on silicon nanoparticles, and design of carbon “pomegranate” structures, containing silicon particles. However, these processes are generally too costly and/or too difficult to scale-up for commercial use.
To improve the cell performance, other electrode materials with higher theoretical capacities are being considered as alternatives for future lithium-ion battery anodes. Silicon is one of the preferred choices due to its high theoretical capacity of about 3600 mAh/g (Li15Si4), almost ten times higher than graphite. However, silicon, while electrochemically active, has poor electron conductivity, and must be connected with electronically conducting particles in a robust, well-dispersed network. Other electrochemically active species might be considered as well, including those that are also conductive. One challenge is providing an anode that retains integrity of the network between the electrochemically active components and electronically conductive particles during the large volume change often found in electrochemically active particles during charge and discharge cycling. For example, both tin and silicon exhibit swelling greater than 300% during repeated charge/discharge cycling, as a result of lithium insertion and extraction taking place in the electrochemical process. Other electrochemically active materials covered in this disclosure are similarly subject to large volume change as a result of insertion and removal of the ions from the electrochemically active material. Examples beyond tin and silicon, include germanium and iron oxide.
One strategy that has started to emerge commercially involves physical mixing of silicon nanoparticles (or submicron particles) with anode-grade graphite microparticles (with typical diameter in the range 10-40 μm), but when the silicon content reaches or exceeds about 5 wt. %, anode performance rapidly degrades due to agglomeration of the silicon particles and disruption of the graphite-silicon connections during cycling. Moreover, the agglomeration can result in the silicon particles becoming separated from the graphite materials, thereby loosing electrical connectivity. In addition, the agglomerated particles cause severe volume changes in the anode and restrict gains in volumetric energy density expected from the addition of silicon, or even leads to reduced performance. These problems are not exclusive to silicon and are problematic for other electrochemically active species given their tendency to agglomerate at higher concentrations and the swelling that occurs during cycling. Thus, the practical limit for adding electrochemically active nanoparticles (such as silicon) to graphite microparticles using current technology is about 5%—above that loading level, the anode suffers severe degradation in performance. Due to the low silicon content that can be practically used with traditional approaches any additional battery capacity is severely limited.
Thus, it remains a serious technical challenge to create anode compositions with silicon content of at least about 5 wt. % such that the silicon particles are non-agglomerating.